1. Field of the Invention
The present invention relates to platens for hydroforming finished plastic sheet material and, more particularly, to a seal used with a blow down platen.
2. Description of Related Art
The process of hydroforming has been practiced for many years. It includes the use of a flexible diaphragm disposed in a cavity of a platen for forcing sheet material to conform with the configuration of a mold. Hydraulic fluid, acting upon the diaphragm, causes the diaphragm to force the sheet material disposed intermediate the diaphragm and the mold to conform with the mold. Because substantial pressures, sometimes exceeding 5,000 psi, may be present, leakage of the hydraulic fluid about the edges of the diaphragm and onto to mold or sheet material to be formed may occur. In hydroforming operations involving a subsequent step in fabricating a product, cleaning is the norm and leakage of hydraulic fluid onto the product molded is of minor concern. When the product molded is to be a finished product, contamination of the product by leaking hydraulic fluid creates a significant problem of requiring an additional cleaning step and possibly compromising the product. To avoid the use of hydraulic fluid and the possibility of contamination of a molded product resulting from leakage of the hydraulic fluid, a hydroforming process known as a "blow down" process has been developed. In this process, the platen includes a cavity conforming in size and configuration with the mold and the sheet material to be molded. After the platen is placed against the table surrounding the mold to close the cavity, a source of high pressure air or gas is channeled into the cavity to create a high pressure environment within the cavity. The high pressure forces the sheet material to be formed against the mold and into conformance with the mold.
The pressure in the cavity of the platen may be maintained by a tight metal to metal fit between the surface of the platen circumscribing the cavity with the sheet material extending thereunder or with the underlying table. A seal of this type is a relatively poor seal and leakage of the air or gas from within the cavity is the norm. To enhance the seal around the cavity in the platen, an O-ring disposed in a groove in the platen has been used. Such an O-ring provides a better seal but the O-ring may easily become damaged due to creep upon application of pressure within the cavity. The integrity of the O-ring may also be compromised by wear upon the O-ring due to lateral movement of the platen relative to the underlying table or sheet material. While an O-ring is relatively inexpensive and easily replaceable, the lost downtime during replacement may be expensive because of the reduced production rate. As the sealing capability of the O-ring deteriorates, the pressures present within the cavity necessary to perform the hydroforming function may be inadequate to form properly the products. The resulting loss of products or shipment of defectively formed products may be costly in the short term and result in loss of business opportunities in the long term.